Fuel cell system to power a portable computing device

ABSTRACT

The disclosed embodiments relate to the design of a portable and cost-effective fuel cell system for a portable computing device. This fuel cell system includes a fuel cell stack which converts fuel into electrical power. It also includes a fuel source for the fuel cell stack and a controller which controls operation of the fuel cell system. The fuel system also includes an interface to the portable computing device, wherein the interface comprises a power link that provides power to the portable computing device, and a bidirectional communication link that provides bidirectional communication between the portable computing device and the controller for the fuel cell system.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 to U.S.Provisional Patent Application No. 61/355,393 filed 16 Jun. 2010,entitled “Portable Hydrogen Fuel Cell System” by inventors Bradley L.Spare, Vijay M. Iyer, Jean L. Lee, Gregory L. Tice, Michael D. Hillmanand David I. Simon (Attorney Docket No. APL-P9535USP1).

BACKGROUND

1. Field

The disclosed embodiments generally relate to systems that use fuelcells to provide electrical power. More specifically, the disclosedembodiments relate to a fuel cell system which is designed to provideelectrical power to a portable computing device.

2. Related Art

Our country's continuing reliance on fossil fuels has forced ourgovernment to maintain complicated political and military relationshipswith unstable governments in the Middle East, and has also exposed ourcoastlines and our citizens to the associated hazards of offshoredrilling. These problems have led to an increasing awareness and desireon the part of consumers to promote and use renewable energy sources.For example, the Electronic Product Environmental Assessment Tool(EPEAT) is presently used to produce data that helps consumers evaluatethe environmental friendliness of electronic products. Moreover, theEPEAT score for an electronic product can be increased by providing arenewable energy source for the product.

As a consequence of this increased consumer awareness, electronicsmanufacturers have become very interested in developing renewable energysources for their products, and they have been exploring a number ofpromising renewable energy sources such as hydrogen fuel cells. Hydrogenfuel cells have a number of advantages. Such fuel cells and associatedfuels can potentially achieve high volumetric and gravimetric energydensities, which can potentially enable continued operation of portableelectronic devices for days or even weeks without refueling. However, itis extremely challenging to design hydrogen fuel cell systems which aresufficiently portable and cost-effective to be used with portableelectronic devices.

SUMMARY

The disclosed embodiments relate to the design of a portable andcost-effective fuel cell system for a portable computing device. Thisfuel cell system includes a fuel cell stack which converts fuel intoelectrical power. It also includes a fuel source for the fuel cell stackand a controller which controls operation of the fuel cell system. Thefuel system also includes an interface to the portable computing device,wherein the interface comprises a power link that provides power to theportable computing device, and a bidirectional communication link thatprovides bidirectional communication between the portable computingdevice and the controller for the fuel cell system.

In some embodiments, the fuel source is a fuel cartridge which isdetachably affixed to the fuel cell system.

In some embodiments, the fuel source comprises sodium borohydride andwater.

In some embodiments, the fuel system includes a fan (or fans) configuredto produce an air flow (or air flows) that: supply oxygen to the fuelcell system; and provide cooling for the fuel cell system.

In some embodiments, the fuel cell system includes an internalrechargeable battery.

In some embodiments, the fuel cell system includes a firstdirect-current-to-direct-current (DC/DC) converter, which converts avoltage from the fuel cell stack into a battery voltage for the internalrechargeable battery.

In some embodiments, the fuel cell system includes a second DC-to-DCconverter, which converts the battery voltage to a computing-devicevoltage which is used to power the computing device.

In some embodiments, the bidirectional communication link communicates:fuel cell state information from the fuel cell system to the portablecomputing device; and fuel cell control information from the portablecomputing device to the fuel cell system.

In some embodiments, the fuel cell state information specifies one ormore of the following: how much power is available from the fuel cellsystem; a state-of-charge of an internal rechargeable battery within thefuel cell system; a temperature of the fuel cell stack; a pressure at aninlet of the fuel cell stack; a pressure at an outlet of the fuel cellstack; cell voltages for individual cells in the fuel cell stack; howmuch fuel remains in the fuel source; and certification information forthe fuel cell system.

In some embodiments, the fuel cell control information specifies one ormore of the following: a request for a specified amount of power fromthe fuel cell system; a reactant rate in the fuel source; a fuel cellstack current to be pulled off the fuel cell stack; a speed of a fanwithin the fuel stack system; and a command to run diagnostics for thefuel cell system.

In some embodiments, the fuel cell state information and the fuel cellcontrol information are used in one or more feedback-control loops toactively control one or more operating parameters of the fuel cellsystem.

In some embodiments, the bidirectional communication link alsocommunicates: computing-device state information from the portablecomputing device to the fuel cell system; and computing-device controlinformation from the fuel cell system to the portable computing device.

In some embodiments, the computing-device state information specifiesone or more of the following: a power requirement for the portablecomputing device; and a state-of-charge of a rechargeable battery withinthe portable computing device.

In some embodiments, the computing-device control information specifies:a power state for the portable computing device, wherein the portablecomputing device uses the power state to control power usage ofcomponents within the portable computing device.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates a fuel cell system in accordance with the disclosedembodiments.

FIG. 1B illustrates how a fuel cell system can be connected to aportable computing device in accordance with the disclosed embodiments.

FIG. 2A illustrates details of the internal structure of a fuel cellsystem in accordance with the disclosed embodiments.

FIG. 2B illustrates a fuel cell system which uses two DC/DC convertersin accordance with the disclosed embodiments.

FIG. 3 presents a flow chart illustrating how a portable computingdevice can control a fuel cell system in accordance with the disclosedembodiments.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the disclosed embodiments, and is provided inthe context of a particular application and its requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other embodiments and applications without departing fromthe spirit and scope of the disclosed embodiments. Thus, the disclosedembodiments are not limited to the embodiments shown, but are to beaccorded the widest scope consistent with the principles and featuresdisclosed herein.

The data structures and code described in this detailed description aretypically stored on a non-transitory computer-readable storage medium,which may be any device or medium that can store code and/or data foruse by a computer system. The non-transitory computer-readable storagemedium includes, but is not limited to, volatile memory, non-volatilememory, magnetic and optical storage devices such as disk drives,magnetic tape, CDs (compact discs), DVDs (digital versatile discs ordigital video discs), or other media capable of storing code and/or datanow known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in anon-transitory computer-readable storage medium as described above. Whena computer system reads and executes the code and/or data stored on thenon-transitory computer-readable storage medium, the computer systemperforms the methods and processes embodied as data structures and codeand stored within the non-transitory computer-readable storage medium.Furthermore, the methods and processes described below can be includedin hardware modules. For example, the hardware modules can include, butare not limited to, application-specific integrated circuit (ASIC)chips, field-programmable gate arrays (FPGAs), and otherprogrammable-logic devices now known or later developed. When thehardware modules are activated, the hardware modules perform the methodsand processes included within the hardware modules.

Fuel Cell System

FIG. 1A provides an external view of a portable fuel cell system 100 inaccordance with the disclosed embodiments. This portable fuel cellsystem 100 includes a fuel cell housing 101, which contains a powermodule with a fuel cell stack that is described in more detail belowwith reference to FIG. 1B. Fuel cell housing 101 is configured toreceive a detachable fuel cell cartridge 104, which contains a suitablefuel, such as sodium borohydride (NaBH₄). Moreover, fuel cell housing101 can provide power to a portable electronic device through a specialinterface 110.

FIG. 1B illustrates some of the internal structures of fuel cell system100 in accordance with the disclosed embodiments. More specifically,fuel cell system 100 includes a fuel cell stack 102 which produceselectrical power by converting a source fuel, such as hydrogen or ahydrocarbon, into an electric current and a waste product. Fuel cellstack 102 comprises a stack of fuel cells, wherein each a fuel cellcontains an anode, a cathode, and an electrolyte between the anode andcathode. Electricity may be generated by two chemical reactions withinthe fuel cell. First, a catalyst at the anode oxidizes the fuel toproduce positively charged ions and negatively charged electrons. Theelectrolyte may allow ions from the oxidation process to pass through tothe cathode while blocking passage of the electrons. The electrons maythus be used to drive a load connected to the fuel cell beforerecombining with the ions and a negatively charged atom (e.g., oxygen)at the cathode to form a waste product such as carbon dioxide and/orwater.

The fuel cells within fuel cell stack 102 may include electrochemicalcells that convert a source fuel into electric current and a wasteproduct. For example, the fuel cells may be proton exchange membrane(PEM) fuel cells that use hydrogen as a fuel. The hydrogen may becatalytically split into protons and electrons at the anode of each PEMfuel cell. The protons may pass through an electrically insulatingmembrane electrode assembly (MEA) to the cathode of the PEM fuel cell,while the electrons may travel through a load to the cathode. Theprotons and electrons may then react with oxygen atoms at the cathode toform water molecules as a waste product. Alternatively, the fuel cellsmay correspond to solid oxide fuel cells, molten carbonate fuel cells,direct methanol fuel cells, alkaline fuel cells, and/or other types offuel cells.

Because individual fuel cells may generate a voltage (e.g., 0.5-0.7volts for PEM fuel cells) which is too low to drive some components in aportable electronic device (e.g., processors, peripheral devices,backlights, displays, Universal Serial Bus (USB) ports, etc.), the fuelcells may be electrically connected in a series configuration. Forexample, a set of 25 PEM fuel cells may be connected in series withinfuel cell stack 102 to increase the voltage of fuel cell stack 102 toroughly 12.5-17.5 volts. This increased voltage may then be used todrive components with operating voltages which are at or below thevoltage of fuel cell stack 102.

Power from fuel cell stack 102 feeds into circuitry 106 that performscontrol functions and performs direct-current (DC)-to-DC conversionoperations before the power feeds through interface 110 to powerportable electronic device 120. The power can also be directed to aninternal rechargeable battery 108, which is configured to store excesspower generated by fuel cell stack 102. Note that internal battery 108can also be used to power a portable electronic device during atransient period when fuel cell stack 102 is preparing to produce power.Also note that instead of using rechargeable battery 108, other energystorage methods can be used, such as Super Capacitors or Lithium-IonCapacitors. In all of these methods, the system simply stores the powerin a convenient location to facilitate meeting the subsequentinstantaneous power requirements of portable electronic device 120.

FIG. 1B also illustrates how fuel cell system 100 can be connected to aportable electronic device 120 through a special interface 110. Thisspecial interface 110 includes: (1) a power link that provides power 112to the portable computing device, and (2) a bidirectional communicationlink that provides bidirectional communication 114 between the portablecomputing device and the controller for the fuel cell system. Thisbidirectional communication link enables the portable electronic deviceto control various aspects of the operation of portable fuel cell system100 as will be described in more detail below.

Note that interface 110 may provide physical links for both power andcommunication. However, in an alternative embodiment, interface 110 mayprovide a single physical connection for the power link and a wirelessbi-directional data link. In other embodiments, the power link may alsobe wireless.

Portable electronic device 120 may correspond to a laptop computer,mobile phone, personal digital assistant (PDA), portable media player,digital camera, and/or other type of compact electronic device. Forexample, portable electronic device 120 may include a processor 126, amemory 124 and a display 128, which are all powered by a power source122. Power source 122 includes a controller 125 which selectivelyprovides power from an internal rechargeable battery 127, or from anexternal source, such as fuel cell system 100.

In an alternative configuration, portable fuel cell system 100 can be“daisy-chained” so that it is connected to another fuel cell systemwhich may or may not be connected in turn to another computer system orcomputing device. Moreover, portable fuel cell system 100 can alsooperate as a standalone device, wherein it operates to charge upinternal battery 108.

Internal Structure of a Fuel Cell System

FIG. 2A illustrates details of some of the internal structure of a fuelcell system 100 in accordance with the disclosed embodiments. Theillustrated fuel cell system 100 includes a fuel cartridge 104 thatplugs into a power module 200. Power module 200 provides power to aportable computing device through a MagSafe™ connector which is coupledto the end of an interface cable 110. (Recall that interface cable 110includes a power link as well as a bidirectional communication link.)Note that the connector on interface cable 110 is not limited to aMagSafe™ connector, and in general can include any type of connectorthat provides power and bi-directional communication, such as a USBconnector or a 30-pin iPod™ connector. Also note that power module 200is located within a fuel cell housing, such as fuel cell housing 101illustrated in FIG. 1A.

Fuel cartridge 104 is comprised of a number of components, which dependon the nature of the fuel. For example, if the hydrogen is produced by ahydrolysis reaction, the fuel cartridge contains components that mayinclude (in addition to a hydrogen-containing substance) anothersubstance (or substances) that chemically react with thehydrogen-containing substance to release hydrogen. To support ahydrolysis reaction, the fuel cartridge can also include: (1) a feed orpump mechanism to enable the substances to combine to produce hydrogen;(2) a metering mechanism to allow for the correct ratio of substancesfor optimal hydrogen production; (3) a heat-dissipation mechanism (suchas a fan) if the hydrogen-producing reaction is highly exothermic; (4)any filters needed to maintain the requisite purity and/or physicalconsistency of the reactants; and (5) a mechanism for containing anywaste product that may result from the hydrogen-producing reaction.Exemplary fuels that can be used with a hydrolysis reaction can include:Sodium Borohydride, Sodium Silicate, Lithium Hydride, Magnesium Hydride,Lithium Borohydride and Lithium Aluminum Hydride.

Moreover, if hydrogen is produced by a thermolysis technique, the fuelcartridge may include (in addition to the hydrogen-containing substance)a heater that heats the hydrogen-containing substance to a sufficientlyhigh temperature to liberate hydrogen. It may also contain a structurefor thermally insulating the heater, and a structure for containing anywaste product that may result. Exemplary fuels that can be used with athermolysis technique can include: Aluminum Hydride, Amine BoraneComplexes (e.g., Ammonia Borane), Hydrocarbons (e.g., Methanol), LithiumAluminum Hydride, Magnesium Borohydride, and a MagnesiumBorohydride-Amine complex.

The fuel may also take the form of pure hydrogen (e.g., compressedhydrogen gas or liquid hydrogen) in which case the fuel cartridge maycontain components such as a metering device (e.g., a valve) and apressure gauge. Ideally, the fuel has a relatively low life cycle carbonfootprint, is not toxic, and generates a waste product that is amenableto being repeatedly re-charged with hydrogen and is not toxic.

More specifically, fuel cartridge 104 is comprised of a number ofcomponents, including fuel and related components 228. During operation,the fuel and related components 228 create hydrogen gas which passesthrough a filter 236 before feeding into an H₂ inlet 240 in power module200. Operations within fuel cartridge 104 are generally controlled by anEEPROM 234, which communicates with master control board 212 in powermodule 200 through an I²C bus 258. A temperature sensor 226 within fuelcartridge 104 determines a temperature of the fuel cartridge 104 andcommunicates a temperature value T_(CARTRIDGE) 257 to EEPROM 234. Inaddition, a cartridge fan 202 within power module 200 pulls a coolingair flow 270 through fuel cartridge 104. Fuel cartridge 104 alsoincludes a pressure relief device (PRD), such as a valve, which ventshydrogen gas (PRD H₂ out 238) if too much hydrogen builds up within fuelcartridge 104.

The flow of hydrogen through fuel cell system 100 is illustrated by thedashed lines. Hydrogen gas which is generated by fuel cartridge 104passes through a pressure sensor 218 in master control board 212 beforefeeding into fuel cell stack 102. Fuel cell stack 102 also includes atemperature sensor 216, which provides an exhaust temperatureT_(EXHAUST) to master control board 212. Excess hydrogen (along withnitrogen and water) exits fuel cell stack through H₂ outlet 241 andfeeds through a pressure sensor 220 in master control board 212 beforefeeding into a passive purge valve 222. Passive purge valve 222 ventsthe excess hydrogen, nitrogen and water through purge output 246.

Fuel cell stack 102 generates power from the hydrogen gas. Morespecifically, voltage outputs V₁, . . . , V_(N) from individual cellswithin fuel cell stack 102 feed into master control board 212, whichdirects power from these voltage outputs into either: internal battery108 through V_(2S), V_(S) and GND connections; or into MagSafe™ board224 though PWR_(2S) and GND connections. Internal battery 108 can storethe power received from fuel cell stack 102, whereas MagSafe™ board 224can direct the power to a portable computing device through interface110 and MagSafe™ connector 225. Master control board 212 controlsinternal battery 108 through a battery management unit (BMU) 206, whichmonitors a temperature T_(B) from internal battery 108.

Master control board 212 also independently controls two or more fans,including cartridge fan 202 and stack fan 204. More specifically, mastercontrol board 212 controls cartridge fan 202 by providing power and apulse-width modulated (PWM) signal 250 to cartridge fan 202. During thisprocess, control board 212 receives a tachometer signal TACH 252 fromcartridge fan 202 which indicates a speed of cartridge fan 202. Asmentioned above, during operation cartridge fan 202 pulls cooling airthrough fuel cartridge 104. Similarly, master control board 212 controlsstack fan 204 by providing power and a pulse-width modulated (PWM)signal 254 to stack fan 204. Control board 212 also receives atachometer signal TACH 256 from stack fan 204 which indicates a speed ofstack fan 204. During operation, stack fan 204 pulls cooling air acrossmaster control board 212 and through fuel cell stack 102.

Master control board 212 is also coupled to a battery indicator light(BIL) board 214 through an I²C link 260. To determine a state-of-chargeof internal battery 108, a user presses an associated BIL button 213. Inresponse to this button press, BIL board 214 communicates with BMU 206within master control board 212 to determine a state-of-charge ofinternal battery 108, and then outputs a pattern on BIL LEDs 211,wherein the pattern indicates the determined state-of-charge.

DC/DC Conversion Process

FIG. 2B illustrates how the fuel cell system can use two DC/DCconverters in accordance with the disclosed embodiments. Duringoperation, master control board 212 receives power from fuel cell stack102 and converts the power using a first DC/DC converter 280 into abattery voltage which is suitable for charging internal battery 108.Next, a second DC/DC converter 282 converts the battery voltage into avoltage suitable for powering a portable electronic device, and thisvoltage is fed into MagSafe™ board 224, which feeds the power to theportable computing device through interface 110.

System Operation

Normal operation of the system begins when the fuel cell system 100 isattached to a host, such as portable electronic device 120. If thestate-of-charge of the internal battery in fuel cell system 100 is in anominal state (between high and low state-of-charge thresholds), powerdelivery to the host begins. The control system then enters aninitialization state and begins requesting fuel from the cartridge.

The cartridge responds by beginning its fuel generation process. Duringthis process, hydrogen may be provided directly from a source of purehydrogen (such as from a vessel containing compressed hydrogen gas), orit may be generated via thermolysis, hydrolysis, electrolysis,reformation, etc. As hydrogen is generated and transported to the powermodule, the cells in the fuel cell stack experience a voltage rise totheir open circuit voltage (OCV). After the voltages cross a threshold,the controller begins to draw small amounts of current.

When this current does not excessively result in depression of the cellvoltages from OCV, the system transitions into a “run” state. In the runstate, hydrogen enters the fuel cell and is converted to current andheat. The oxygen for the reaction is supplied by the stack fan fromambient air, and heat is exchanged by controlling the fan to maintainthe stack at a constant temperature.

Current from the stack is converted to the voltage of the internalbattery and stored. The pressure control loop maintains stack outletpressure at a set point by controlling the DC/DC input current andcharging/discharging the internal battery as necessary. The output ofthe internal battery charger goes through another DC/DC conversion andthen powers the host computer.

In the case of a fuel cartridge where hydrogen is produced by anexothermic reaction, the controller maintains the cartridge temperatureat its set point using a cartridge fan.

During normal stack operation, nitrogen and water must be periodicallypurged from the anode side. The system transitions to the purge statewhen the mean cell voltage drops below a threshold. During a “purge”state, the load on the stack is removed, resulting in an anode pressurerise. When the anode pressure exceeds the burst pressure of the passivepurge valve, the purge valve opens. At the termination of the purge, thestack is loaded, and the pressure of the stack is returned to thenominal operating pressure. The passive purge valve closes when thepressure decreases below the closing pressure of the valve.

Many of the above-described operations of fuel cell system 100 can becontrolled through communications between fuel cell system 100 andportable electronic device 120 as is described in more detail below withreference to the flow chart in FIG. 3.

Controlling a Fuel Cell System from a Portable Electronic Device

FIG. 3 presents a flow chart illustrating how a portable computingdevice can control a fuel cell system in accordance with the disclosedembodiments. The left-hand column of FIG. 3 lists actions performed byfuel cell system 100 and the right-hand column lists actions performedby portable electronic device 120. During operation, fuel cell system100 sends fuel cell state information to portable electronic device 120,wherein the fuel cell state information is sent through an interfacethat comprises: a power link that provides power to the portablecomputing device; and a bidirectional communication link that providesbidirectional communication between the portable computing device andthe fuel cell system (step 302). For example, the fuel cell stateinformation can specify one or more of the following: how much power isavailable from the fuel cell system; a state-of-charge of an internalrechargeable battery within the fuel cell system; a temperature of thefuel cell stack; a pressure at an inlet of the fuel cell stack; apressure at an outlet of the fuel cell stack; cell voltages forindividual cells in the fuel cell stack; how much fuel remains in thefuel source; certification information for the fuel cell system; andidentification information that facilitates identifying an individualfuel cell unit and/or individual fuel cartridges.

Next, portable electronic device 120 receives the fuel cell stateinformation (step 304), and in response generates fuel cell controlinformation based on the received fuel cell state information (step306). For example, the fuel cell control information can specify one ormore of the following: a request for a specified amount of power fromthe fuel cell system; a reactant rate in the fuel source; a fuel cellstack current to be pulled off the fuel cell stack; a speed of a fanwithin the fuel stack system; and a command to run diagnostics for thefuel cell system.

The fuel cell control information can also specify power demandpredictions. For example, the fuel cell control information can specifythat portable computing device 120 expects to require 60 W of power inten minutes time. In another example, as the battery within the portableelectronic device 120 charges up, the fuel cell control information canspecify that the demand for power from the fuel cell system 100 islikely to decrease over time. Providing such power demand predictionsenables fuel cell system 100 to optimize its performance differentlythan if such predictions were not available.

Portable electronic device 120 then transmits the fuel cell controlinformation to fuel cell system 100 through the bidirectionalcommunication link (step 308). Finally, fuel cell system 100 receivesthe fuel cell control information (step 310), and uses the received fuelcell control information to control the fuel cell system (step 312). Theabove-described processes for controlling the fuel cell system caninvolve using one or more feedback-control loops to actively control oneor more operating parameters of the fuel cell system.

The foregoing descriptions of embodiments have been presented forpurposes of illustration and description only. They are not intended tobe exhaustive or to limit the present description to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present description. The scopeof the present description is defined by the appended claims.

1. A fuel cell system for a portable computing device, comprising: afuel cell stack which converts fuel to electrical power; a fuel sourcefor the fuel cell stack; a controller which controls operation of thefuel cell system; and an interface to the portable computing device,wherein the interface comprises, a power link that provides power to theportable computing device, and a bidirectional communication link thatprovides bidirectional communication between the portable computingdevice and the controller for the fuel cell system.
 2. The fuel cellsystem of claim 1, wherein the fuel source is a fuel cartridgedetachably affixed to the fuel cell system.
 3. The fuel cell system ofclaim 1, wherein the fuel source comprises at least one of: sodiumborohydride and water; sodium silicate and water; lithium hydride andwater; magnesium hydride and water; lithium borohydride and water;lithium aluminum hydride and water; aluminum hydride; an amine boranecomplex; a hydrocarbon; lithium aluminum hydride; magnesium borohydride;a magnesium borohydride-amine complex; compressed hydrogen gas; andliquid hydrogen.
 4. The fuel cell system of claim 1, further comprisinga fan within the fuel cell system configured to produce an air flowthat: supplies oxygen to the fuel cell system; and provides cooling forthe fuel cell system.
 5. The fuel cell system of claim 1, furthercomprising an internal rechargeable battery within the fuel cell systemconfigured to level a load for the fuel cell system.
 6. The fuel cellsystem of claim 5, further comprising a first direct-current (DC)-to-DCconverter, which converts a voltage from the fuel cell stack into abattery voltage for the internal rechargeable battery.
 7. The fuel cellsystem of claim 6, further comprising a second DC-to-DC converter, whichconverts the battery voltage to a computing-device voltage which is usedto power the computing device.
 8. The fuel cell system of claim 1,wherein the bidirectional communication link communicates: fuel cellstate information from the fuel cell system to the portable computingdevice; and fuel cell control information from the portable computingdevice to the fuel cell system.
 9. The fuel cell system of claim 8,wherein the fuel cell state information specifies one or more of thefollowing: how much power is available from the fuel cell system; astate-of-charge of an internal rechargeable battery within the fuel cellsystem; a temperature of the fuel cell stack; a pressure at an inlet ofthe fuel cell stack; a pressure at an outlet of the fuel cell stack;cell voltages for individual cells in the fuel cell stack; how much fuelremains in the fuel source; and certification information for the fuelcell system.
 10. The fuel cell system of claim 8, wherein the fuel cellcontrol information specifies one or more of the following: a requestfor a specified amount of power from the fuel cell system; a reactantrate in the fuel source; a fuel cell stack current to be pulled off thefuel cell stack; a speed of a fan within the fuel stack system; and acommand to run diagnostics for the fuel cell system.
 11. The fuel cellsystem of claim 8, wherein the fuel cell state information and the fuelcell control information are used in one or more feedback-control loopsto actively control one or more operating parameters of the fuel cellsystem.
 12. The fuel cell system of claim 1, wherein the bidirectionalcommunication link also communicates: computing-device state informationfrom the portable computing device to the fuel cell system; andcomputing-device control information from the fuel cell system to theportable computing device.
 13. The fuel cell system of claim 12, whereinthe computing-device state information specifies one or more of thefollowing: a power requirement for the portable computing device; and astate-of-charge of a rechargeable battery within the portable computingdevice.
 14. The fuel cell system of claim 12, wherein thecomputing-device control information specifies: a power state for theportable computing device, wherein the portable computing device usesthe power state to control power usage of components within the portablecomputing device.
 15. A method for controlling a fuel cell system for aportable computing device, comprising: controlling the fuel cell systemfrom the portable computing device through an interface which comprises,a power link that provides power to the portable computing device, and abidirectional communication link that provides bidirectionalcommunication between the portable computing device and the fuel cellsystem.
 16. The method of claim 15, wherein controlling the fuel cellsystem involves: receiving fuel cell state information from the fuelcell system through the bidirectional communication link; generatingfuel cell control information based on the received fuel cell stateinformation; and transmitting the fuel cell control information to thefuel cell system through the bidirectional communication link.
 17. Themethod of claim 16, wherein the fuel cell state information specifiesone or more of the following: how much power is available from the fuelcell system; a state-of-charge of an internal rechargeable batterywithin the fuel cell system; a temperature of a fuel cell stack withinthe fuel cell system; a pressure at an inlet of the fuel cell stack; apressure at an outlet of the fuel cell stack; cell voltages forindividual cells in the fuel cell stack; how much fuel remains in a fuelsource for the fuel cell system; and certification information for thefuel cell system.
 18. The method of claim 16, wherein the fuel cellcontrol information specifies one or more of the following: a requestfor a specified amount of power from the fuel cell system; a reactantrate in a fuel source for the fuel cell system; a fuel cell stackcurrent to be pulled off a fuel cell stack within the fuel cell system;a speed of a fan within the fuel stack system; and a command to rundiagnostics for the fuel cell system.
 19. The method of claim 16,wherein controlling the fuel cell system involves using the fuel cellstate information and the fuel cell control information in one or morefeedback-control loops to actively control one or more operatingparameters of the fuel cell system.
 20. The method of claim 15, furthercomprising sending computing-device state information from the portablecomputing device to the fuel cell system.
 21. The method of claim 20,wherein the computing-device state information specifies one or more ofthe following: a power requirement for the portable computing device;and a state-of-charge of a rechargeable battery within the portablecomputing device.
 22. The method of claim 15, further comprisingreceiving computing-device control information from the fuel cell systemat the portable computing device.
 23. The method of claim 22, whereinthe computing-device control information specifies: a power state forthe portable computing device, wherein the portable computing deviceuses the power state to control power usage of components within theportable computing device.
 24. A method for controlling a fuel cellsystem that powers a portable computing device, comprising: sending fuelcell state information from the fuel cell system to the portablecomputing device, wherein the fuel cell state information is sent to theportable computing device through an interface that comprises, a powerlink that provides power to the portable computing device, and abidirectional communication link that provides bidirectionalcommunication between the portable computing device and the fuel cellsystem; receiving fuel cell control information from the portablecomputing device through the bidirectional communication link in theinterface; and using the received fuel cell control information tocontrol the fuel cell system.
 25. The method of claim 24, wherein thefuel cell state information specifies one or more of the following: howmuch power is available from the fuel cell system; a state-of-charge ofan internal rechargeable battery within the fuel cell system; atemperature of a fuel cell stack within the fuel cell system; a pressureat an inlet of the fuel cell stack; a pressure at an outlet of the fuelcell stack; cell voltages for individual cells in the fuel cell stack;how much fuel remains in a fuel source for the fuel cell system; andcertification information for the fuel cell system.
 26. The method ofclaim 25, wherein the fuel cell control information specifies one ormore of the following: a request for a specified amount of power fromthe fuel cell system; a reactant rate in a fuel source for the fuel cellsystem; a fuel cell stack current to be pulled off a fuel cell stackwithin the fuel cell system; a speed of a fan within the fuel stacksystem; and a command to run diagnostics for the fuel cell system.
 27. Aportable computing device that controls a fuel cell system, comprising:a processor; a memory; and an interface to the fuel cell system, whereinthe interface comprises, a power link that provides power to theportable computing device from the fuel cell system, and a bidirectionalcommunication link that provides bidirectional communication between theportable computing device and the fuel cell system; wherein the portablecomputing device is configured to control the fuel cell system throughthe bidirectional communication link.
 28. The portable computing deviceof claim 27, wherein while controlling the fuel cell system, theportable computing device is configured to: receive fuel cell stateinformation from the fuel cell system through the bidirectionalcommunication link; and generate fuel cell control information based onthe received fuel cell state information; transmit the fuel cell controlinformation to the fuel cell system through the bidirectionalcommunication link.
 29. The portable computing device of claim 28,wherein the fuel cell state information specifies one or more of thefollowing: how much power is available from the fuel cell system; astate-of-charge of an internal rechargeable battery within the fuel cellsystem; a temperature of a fuel cell stack within the fuel cell system;a pressure at an inlet of the fuel cell stack; a pressure at an outletof the fuel cell stack; cell voltages for individual cells in the fuelcell stack; how much fuel remains in a fuel source for the fuel cellsystem; and certification information for the fuel cell system.
 30. Theportable computing device of claim 28, wherein the fuel cell controlinformation specifies one or more of the following: a request for aspecified amount of power from the fuel cell system; a reactant rate ina fuel source for the fuel cell system; a fuel cell stack current to bepulled off a fuel cell stack within the fuel cell system; a speed of afan within the fuel stack system; and a command to run diagnostics forthe fuel cell system.
 31. The portable computing device of claim 28,wherein while controlling the fuel cell system, the portable computingdevice is configured to use the fuel cell state information and the fuelcell control information in one or more feedback-control loops toactively control one or more operating parameters of the fuel cellsystem.
 32. The portable computing device of claim 27, wherein theportable computing device is also configured to send computing-devicestate information to the fuel cell system.
 33. The portable computingdevice of claim 32, wherein the computing-device state informationspecifies one or more of the following: a power requirement for theportable computing device; and a state-of-charge of a rechargeablebattery within the portable computing device.
 34. The portable computingdevice of claim 27, wherein the portable computing device is alsoconfigured to receive computing-device control information from the fuelcell system.
 35. The method of claim 34, wherein the computing-devicecontrol information specifies: a power state for the portable computingdevice, wherein the portable computing device uses the power state tocontrol power usage of components within the portable computing device.